Age and extended sitting can often kill a fuel pump but what raises a red flag to me is not just the loss of power but the burning smell.
Hopefully that burning smell is not related to someone botching the oil change.
Assuming the oil change has not been botched then a burning smell could be a fluid leak burning off of the exhaust or a bad accessory belt that is slipping badly. I would assume, right or wrong, that checking for leaks or a bad belt would be done while the oil was being changed. I know that I always do this.